Electrically conducting organic polymers have been of scientific and technological interest since the late 1970's. These relatively new materials exhibit the electronic and magnetic properties characteristic of metals while retaining the physical and mechanical properties associated with conventional organic polymers.
Technological application of these polymers are beginning to emerge. Today, conductive polymers and composites such as mentioned above have a broad range of applications including their use as materials for carders of electrically sensitive devices which prevent electrostatic charge (ESC) which may attract airborne particles on critical surfaces and electrostatic discharge (ESD) which may cause device malfunction. In addition, conducting polymers can be used as machine covers for electronic equipment which prevent the ingress or egress of electromagnetic signals in order to meet the guidelines established by the FCC as to the accepted levels of unwanted electrical noise.
The materials currently in use are rendered conductive through the use of conductive fillers like metal, carbon particles or chemicals such as ionic salts. The problems associated with these materials include high cost, sloughing of the filler, dependency on environmental conditions, and a very high surface resistance.
Polyanilines are known to be a class of soluble, processable electrically conducting organic polymers. This family of polymers displays a range of solubilities in organic and aqueous acid solutions. Polyanilines are rendered conducting by treatment with cationic reagents (Lewis acids), most commonly protonic acids. Also the polyaniline can be doped by taking the non-conducting form of the polymer and amine triflate salts (which thermally generate acid) and mildly heating them together in the form of a film or in solution, example Although polyaniline is very inexpensive to produce, some of its physical properties such as the impact strength, tensile strength, etc., may limit the full scope of its uses. The techniques disclosed in prior art references are completely different from the present invention.
One application of the polymer in the prior art uses polydopants, for example, polyimide precursors such as the polyamic acid (--COOH) form (with a high molecular weight as made) as direct dopants for the polyaniline to obtain conducting blends of the two polymers in one step. In the case of polyamic acid, the polyaniline becomes protonated by the polyamic acid.
Also, in the prior art, an anhydride reacted polyaniline is blended with polyimides. By contrast, in another prior art reference, a conductive blend is obtained in a single step due to the interaction between the polydopant (polyamic acid) and the conducting polymer leading to a compatible conducting polymer blend. The resultant blend in the present invention has dispersion at a molecular scale as opposed to the prior art wherein the dispersion is at a much comer scale. The references cited do not disclose formation of a conducting complex or blend with the polyamic acid but rather the polyaniline is reacted with anhydrides first to obtain a product, and thereafter, this product is blended with another polyimide.
Another reference uses polydopants, such as Br.o slashed.nsted acid (protonic) polymers. Examples of such polydopants are: polyacrylic acid, polysulfonic acid, cellulose sulfonic acid, polyamic acid, photosensitive polyamic acid, polyphosphoric acid, acid chloride (--COCl) containing polymers and sulfonyl chloride (--SO.sub.2 Cl) containing polymers. The advantage of the use of such materials are that no external corrosive monomeric or oligomeric dopants are necessary; there is high thermal and electrical stability due to the polymeric counteranion; and there is enhanced processability. It is important to note that because of the interaction of the two polymers as stated above, compatible molecularly mixed blends are formed wherein there is no phase separation. Finally, the solution gels over time which allows the formation of highdraw ratio fibers.
Electrically conducting organic polymers have been of scientific and technological interest because these relatively new materials exhibit the: electronic and magnetic properties characteristic of metals while retaining the physical and mechanical properties associated with conventional organic polymers.
More specifically, polyaniline has become especially important among conductive polymers because its doped conductive form is environmentally stable. Ever since the discovery that the acid salt of polyaniline exhibits electrical conductivity, much effort has been directed to increase the conductivity and tractability of polyaniline.
These efforts have generally centered on the optimization of the water based oxidative coupling of aniline and its derivative and post processing of the polymer obtained by this method. This method is illustrated by the equation as set forth in Reaction I below. ##STR1##
In the oxidative coupling approach, aniline or its derivatives are dissolved in aqueous acid solution and treated with electron transfer oxidizing agents such as ammonium persulfate. The polyaniline thus formed has an electrical conductivity in the range of about 0.1 to 10 S/cm; and because of the presence of free radicals and the mechanism of the coupling reaction, crosslinks and defects are likely to be introduced into the polymer chain which results in reducing its crystallinity and conductivity. The efforts to improve the conductivity of polyaniline has been directed generally toward increasing the crystallinity of the latter by mechanical stretching and/or choice of dopants and additives such as listed above.
Technological application of these polymers are beginning to emerge. These polymers are electrically conductive substituted and unsubstituted polyanilines, substituted and unsubstituted polyparaphenylenes, substituted and unsubstituted polyparaphenylenevinylenes, substituted and unsubstituted polythiophenes, substituted and unsubstituted polyazines, substituted and unsubstituted polyfuranes, substituted and unsubstituted polypyrroles, substituted and unsubstituted polyselenophenes, substituted and unsubstituted polyphenylene sulfides and substituted and unsubstituted polyacetylenes formed from soluble precursors. Blends of these aforementioned polymers are suitable for use as are copolymers made from the monomers used to form these polymers.
The articles entitled Polyaniline; Processability From Aqueous Solutions and Effective Water Vapor on Conductivity to M. Angelopoulos et al., Synthetic Metals, 21 (1987) pp.21-30, and the article entitled Polyaniline: Solutions, Films, Oxidation State to M. Angelopoulos et al., Mol. Cryst. Liq. Cryst. 160-151 (1988), describe a chemically synthesized emeraldine base form of polyaniline which is soluble in various solvents. The emeraldine base is doped by reacting, the emeraldine powder or film with aqueous acid solution for several hours, for example, aqueous acetic acid or aqueous HCl.
Electrically conducting polymers are described in detail U.S. Pat. No. 5,198,153 noted above and U.S. application Ser. No. 08/118,475.
Conducting polymers can be conveniently employed in applications where the use of metal would be too expensive or inappropriate due to processing considerations. Such applications generally require that the physical properties of the interconnect material impart resiliency, high initial and ultimate adhesion as well as corrosion resistance and especially flexibility. The combination of such properties is difficult to achieve with an all metal connection.